Field of the Invention
The present invention concerns methods and systems for acquiring magnetic resonance data, and in particular to methods and systems wherein at least one radio-frequency (RF) pulse is radiated that saturates the magnetization of nuclear spins of fat in a data acquisition region of an examination subject. Sequences or protocols for operating a magnetic resonance apparatus known as TIRM (Turbo Inversion Recovery with a Magnitude display) protocols provide an image with a contrast, which is becoming more popular, for example, for spine imaging. Ideally, when multiple images are combined in a stack in order to collectively produce an image of an anatomical feature such as the spine, each image in the stack should have the same fat saturation, so that when the reconstructed images are displayed, the combined stack does not exhibit differences from slice-to-slice, which detract from the diagnostic quality of the overall image.
Description of the Prior Art
In conventional techniques for data acquisition for this type of image, however, the fat saturation is not homogenous for the entire slice stack.
This is because of the nature of the conventional imaging sequences that are used for this purpose. An example of such a known sequence is shown in FIG. 1 with a sequence proceeding from left to right with increasing time t. After the end of a data acquisition (readout), inversion recovery (IR) pulses are radiated to invert the nuclear spins that existed at the end of the previous acquisition. In the next acquisition, data are read out for different slices S1-1, S1-2, S1-3, etc., from echo trains, each preceded by a fat saturation (FatSat) pulse, which has a flip angle of 140°. A waiting time (inversion time), which typically is approximately 900 to 1,000 ms occurs before data from the first slice is acquired. This type of sequence is known as an interleaved inversion recovery sequence. The radiation of the FatSat pulse prior to each slice acquisition suppresses fat signals originating from the tissue from which the data are acquired.
The flip angle of the FatSat pulse depends on the repetition time thereof, and thus applying the same FatSat pulse (i.e., a FatSat pulse with the same flip angle of 140°) results in the fat signal that occurs being different in different slices.
A similar situation occurs in triggered TSE (Turbo Spin Echo) scans, as shown in the known sequence illustrated in FIG. 2. In such scans, one echo train per slice, for several slices, is detected after a waiting time, defined by the breathing cycle of the patient, in order for the data to be acquired in each acquisition with the patient in the identical breathing state. Due to the necessary waiting time between the successive data acquisitions, the FatSat pulse for the first slices is less effective than for the later slices.